1. Field of the Invention
The invention relates to the field of pulsed combustion devices, and in particular concerns an optimized oil-fired boiler driven by pulse-controlled convection, having a gas/fluid heat exchanger for extracting heat energy from hot exhaust gases.
According to the invention, flapper valves on an air inlet decoupler chamber oscillate the feed air flow. A fuel nozzle injects atomized fuel. Structures located downstream of the air and fuel inlets along the air flow path cooperate by successively diverting flow in different directions to induce turbulence, for example inducing toroidal and then helical flow and corresponding additive eddy currents for good air/fuel mixing. Oscillation is maintained due to the push-pull effect on pressure conditions of alternating expansion and exhaust of the combusting air/fuel mixture, which pressure conditions drive the closing and opening of the flapper valves and contribute to the flow of combustion gases in the draft direction toward the exhaust. The hot exhaust gases pass from the combustion chamber through a heat exchanger tube bundle that is immersed in a water jacket defined between the concentric walls of the combustion chamber and an outer housing of the unit. The tube bundle comprises an array of axially and radially elongated U-shaped tubes in the annular space between the combustion chamber and the outer walls. Each tube is coupled between an inner annular tube manifold plate on the upstream side of the flow path and an outer annular tube manifold plate on the downstream side, with the connections of each tube to the inner and outer manifold plates being angularly advanced such that the array is compactly compressed. The downstream or outer annular tube manifold opens into an exhaust decoupler chamber that is bounded by the outer bell end of the generally cylindrical unit such that the axial end of the housing encloses the gas flow path rather than the water jacket.
2. Related Art
Pulsed combustion devices generally comprise a combustion chamber and one or more exhaust pipes arranged to transfer heat into a forced air heating system or into the water of a boiler. Pulse combustion can involve pulsing the fuel and/or air that is fed to the combustion chamber. A pulse air system can have an inlet air valve leading to the combustion chamber, arranged to resonate at an oscillation frequency determined by the combustion chamber volume, the volume of the exhaust pipes, the length of the exhaust pipes, and other physical parameters (such as the speed of sound).
A pulsed air combustion device can be considered to operate on an oscillatory pressure cycle. In general, the air/fuel charge ignited in the combustion chamber expands as it is heated, and flows into and through the exhaust pipes due to expansion with combustion and heating. Combustion expansion causes a local pressure increase at the area of combustion. There are obstructions placed upstream of the combustion area or chamber, namely closer to the air inlet compared to the draft or flow direction (including at least the inlet flapper valves). The expansion thus moves the gases forward in the draft direction in a higher pressure phase of the combustion cycle. In addition, convective flow of hot gases in the draft direction creates a partial vacuum at the combustion chamber in a lower pressure or relative vacuum phase of the pressure cycle, which assists in drawing a fresh air/fuel charge into the combustion chamber. With correct timing, including oscillation of the air inlet at the required frequency, pulsed combustion ensues and becomes self sustaining. The fresh air/fuel charge can be spontaneously ignited when exposed to still-combusting gases or to the latent heat of the combustion chamber. An initial cycle is established through ignition by a spark, pilot flame or glow plug, and the process is then self sustaining, with an oscillatory pressure fluctuation superimposed on a general flow of gases from the inlet to the outlet.
With appropriate adjustments to the dimensions of the respective chambers and the valves or other means that control the feeding of air and fuel, it is possible to vary parameters including the resonant frequency of the device, the rate of fuel combustion, the ratio of air feed to fuel volume and the like. One objective of such adjustments is to extract as much thermal energy as possible from the fuel used, by complete combustion and efficient thermal energy transfer. There may be other objectives in addition to efficiency, such as maintaining a high rate of thermal energy transfer. Other objectives such as durability versus expense and ease of construction, are also involved. These objectives compete and may involve opposite considerations, such that they are difficult to optimize together.
Pulse combustion technology is advantageous for meeting many of the objectives, especially in water heating units intended for high rates of thermal energy transfer. Pulse boilers also have unique design concerns. For example, such units operate resonantly at low frequency and generate acoustic noise.
Combustion boilers are advantageously compact. It is particularly important to make the device compact if the heat transfer fluid (water) is to be pressurized. If the pressure confined in a vessel is increased, it is obviously necessary to make the walls of that vessel thicker to withstand the additional pressure. Moreover, at a given pressure, if the span of a pressure confining wall of a vessel is increased, that is if a wall is widened and/or lengthened to encompass a greater area, it is also necessary to make that wall thicker. However, the relationship of required wall thickness versus wall span in a pressure vessel is a geometric relationship. Thus it is particularly important to keep the design compact.
With thermal transfer between combustion gas and a pressurized fluid, a relatively small heat transfer surface provides good efficiency of heat transfer on the fluid side. A water jacket can substantially or only partly enclose a combustion chamber and its associated exhaust gas conduits. Heat transfer fins and surfaces can be employed to increase the rate of thermal transfer by increasing the surface contact area, particularly on the exhaust or flue gas side of the heat exchange surfaces.
The extent of thermal transfer normally can be increased by more completely and deeply embedding the combustion and exhaust portions of such a unit in the water jacket. For example, the combustion chamber can be placed well within a large water jacket volume that encloses the combustion chamber on all sides. The exhaust pipes can be arranged to carry the hot exhaust gases a long distance through the water jacket by which heat is extracted, and the water or other coolant can be kept at a relatively low temperature such that the ultimate exhaust to the ambient atmosphere is relatively cool compared to the combustion temperature. However, these aspects add to the overall size and complexity of the unit. Lengthening the exhaust pipes also lowers the resonant frequency of the system. Such changes may increase thermal energy transfer efficiency but can have other drawbacks and advantageously should be optimized.
A pulse combustion furnace for liquid or gaseous fuel is disclosed in U.S. Pat. No. 4,995,376--Hanson. This patent discloses and claims certain structural details that preferably are applied to the boiler of the present invention. The '376 Hanson patent is hereby incorporated by reference for its disclosure of structures in common with the preferred embodiments of the present invention.
U.S. Pat. No. 5,242,294--Chato discloses a pulse combustion boiler which operates at a high resonant frequency (i.e., 440 Hz). U.S. Pat. No. 4,951,706--Kardos discloses a flapper air check valve for oscillating the feed air to a combustion device. The disclosures of these references are also hereby incorporated.
There is a need for a high-capacity pulse combustion boiler, having a compact design and an acceptable acoustic noise level, which is optimized for efficiency in its rate of heat transfer, and is also characterized by design features that contribute to efficiency while maintaining a compact size. Preferably, this should be accomplished without contributing unduly to the cost and complexity of the device and with minimum requirements for unduly heavy or complex construction.